Secondly, you need to make it clear in the plugin description that activating the plugin will immediately change the 404 pages on the site, and that the plugin will get these pages from notfound.org. This way users are not surprised that it "turns itself on", so to speak.

Finally, having a settings page with no actual settings is kind of pointless. You should just turn off that menu option and remove that wording from the readme until you have something for them to actually configure. You can leave the code, then re-enable it later when you actually have settings on there.